DESCRIPTION: In keeping with the tradition of this laboratory, the application consists of four relatively independent projects, connected here because they involve actin as opposed to myosin. The projects are as follows: (1) Characterize the effects of nucleotide and polyproline on actin-profilin interactions. Search for proteins that bind to the polyproline binding site of profilin. Use profilin mutants to test which biochemical interactions are important in vivo in fission yeast. (2) The Arp2/3 complex, identified as binding to profilin. Aims are to sequence the other five proteins, to determine the structure of the complex and interactions between the subunits, and to reconstitute the complex from purified components. (3) To characterize the binding of actophorin (a cofilin/ADF relative) to actin, especially the severing of actin filaments by actophorin and to determine the effects in vivo of changing the actophorin concentration. (4) To test further a model in which alpha-actinin functions as a dynamic crosslinker of actin filaments in vitro.